In the operation of a gas or oil burning furnace, a single fluid line controlled by a solenoid valve is normally used to deliver the fuel to the burner of the furnace. The opening and closing of this valve is, in turn, controlled by a thermostatic switch. Only a single volume rate of flow of fuel can be realized with this fuel delivery system. When the fuel is delivered to the furnace burner upon closing of the thermostatic switch, the fuel is ignited and continues to burn until the thermostatic switch is opened, at which time the valve is closed and fuel flow ceases. The valve will remain closed until the thermostatic switch is again closed, whereupon the process is repeated. During the time in which the valve is closed, the heat energy in the space heated by furnace is progressively dissipated until the thermostatic switch closes, whereupon the valve opens and the furnace is operated to keep the space at a desired temperature as sensed by the thermostatic switch.
It has been found that such a space can be heated at a lower rate to conserve fuel without causing too much discomfort to the occupants of the space. If the volume rate of flow of fuel to the furnace is lowered, fuel costs can be decreased yet the heating of a space can be done efficiently even though it takes a longer time to produce the same heating results as with a higher volume rate of flow of fuel. Because of this finding, a need has arisen for improved apparatus and method for controlling the flow of fuel to a furnace in a manner to achieve a conservation of fuel without seriously causing discomfort to occupants of the space heated by the furnace.